Printing method and system by gelatin coagulation

ABSTRACT

A method and a system adapted for high speed image reproduction or surface coating by electro-osmosis drying of a colloid. A thin layer containing a colloid (gelatin or albumin), water and a salt selected from chlorides and sulfates, is sandwiched between at least one pair of opposite electrodes, and an electrical bias on the latter causes the colloid to coagulate and adhere to one electrode, thereby forming a coating or an imprint. A coating of highly uniform thickness can be obtained and which may be varied by adjusting the quantity of current passed through the layer. An image may be electrically produced by sandwiching such layer between an electrode means, and a plurality of electricallyinsulated juxtaposed electrodes. The temperature of the layer is controlled so that the gelatin remains liquid and spatially selected ones of the plurality of electrodes are electrically biased with respect to the electrode means to pass electric pulses through the layer, whereby coagulation adherence of gelatin is produced at points adjacent to each biased electrode. The coagulated gelatin adheres to the positive electrodes and the remaining non-coagulated liquid layer is thereafter removed, thus forming a printing plate. The pulsations of the electrical signal may be of different amplitudes from one electrode to the other, in accordance with the shades or light of the image to be reproduced, whereby to produce a corresponding variation in the amount of coagulated gelatin resulting in a printing plate which reproduces the half-tones of the image. A very thin coating can be obtained of the order of several thousand Angstroem units; thus, polychromatic reproduction can be obtained using the principle of interferential light reflection.

'United States Patent [191 Castegnier [451 July 1, 1975 PRINTING METHOD AND SYSTEM BY GELATIN COAGULATION [76] Inventor: Adrien Castegnier, 325 Melbourne St., Mount Royal, Quebec, Canada [22] Filed: June 6, 1973 [21] Appl. No.: 367,589

[52] 11.5. C1 204/180 R; 117/934; 204/2; 204/130', 204/228; 204/274; 204/300 [51] int. C1 B0lk 1/00;B01k 5/00; BOlk 5/02 [58] Field of Search 204/180 R, 181, 2, 130, 204/181 PE, 228, 274, 300; 117/37, 93, 93.4

[56] References Cited UNITED STATES PATENTS 1,577,642 3/1926 Jenny et al.... 204/180 R X 1,739,766 12/1929 Morris 204/180 R X 1,796,584 3/1931 Volmer..... 204/180 R X 2,107,294 2/1938 Griswold 204/181 X 2,906,682 9/1959 Fahnoe et al. 204/181 3,010,883 11/1961 Johnson ct al.... 204/2 X 3,079,859 3/1963 204/2 X 3,145,156 8/1964 Oster 204/180 R Primary Examiner-John H. Mack Assistant ExaminerA. C. Prescott [57] ABSTRACT A method and a system adapted for high speed image (gelatin or albumin), water and a salt selected from chlorides and sulfates, is sandwiched between at least one pair of opposite electrodes, and an electrical bias on the latter causes the colloid to coagulate and adhere to one electrode, thereby forming a coating or an imprint. A coating of highly uniform thickness can be obtained and which may be varied by adjusting the quantity of current passed through the layer. An image may be electrically produced by sandwiching such layer between an electrode means, and a plurality of electrically-insulated juxtaposed electrodes. The temperature of the layer is controlled so that the gelatin remains liquid and spatially selected ones of the plurality of electrodes are electrically biased with respect to the electrode means to pass electric pulses through the layer, whereby coagulation adherence of gelatin is produced at points adjacent to each biased electrode. The coagulated gelatin adheres to the positive electrodes and the remaining non-coagulated liquid layer is thereafter removed, thus forming a printing plate. The pulsations of the electrical signal may be of different amplitudes from one electrode to the other, in accordance with the shades or light of the image to be reproduced, whereby to produce a corresponding variation in the amount of coagulated gelatin resulting in a printing plate which reproduces the halftones of the image. A very thin coating can be obtained of the order of several thousand Angstroem units; thus, polychromatic reproduction can be obtained using the principle of interferential light reflection.

13 Claims, 8 Drawing Figures SHEET MODULATOR PRINTING METHOD AND SYSTEM BY GELATIN COAGULATION This invention relates to image or symbol reproduction and to surface coating and, in particular. to an electric image or symbol reproduction and to a surface coating method and a system therefor of the type wherein solidification is produced in an electrically reactive liquid state layer.

The method and system of the present invention are suitable for use in the field of-surface coating wherein a coating of highly uniform thickness is desired and also in various reproduction fields, such as image or symbol reproduction, printing, photography, photocopy, computer printing outputs, radiography, engraving, photo engraving, printing plate preparation in lithography, silk screen and printed circuits.

The present invention consists essentially of applying the principle of electro-osmosic drying of a colloid to surface coating and image or symbol reproduction. The method of the invention consists in passing direct current through a thin layer of a liquid state colloid containing an electrolyte by means of opposite electrodes in contact with the layer, resulting in the coagulation and adherence of part of the colloid to one of the elec' trodes and thereafter removing any excess liquid state colloid. Coagulation and adherence of the colloid to the electrode is believed to result from electro-osmosis; the water trapped in the liquid state colloid migrates under direct current towards one electrode; the colloid acts as a porous filter and dries out upon migration of water and adheres to the other electrode.

The phenomenon is slow when no electrolyte is present but is considerably accelerated in the presence of a suitable electrolyte, whereby ultra-fast coating or image reproduction can be obtained. In the presence of the electrolyte, it has been found that the speed of colloid coagulation only depends on the duration of the electric pulse applied to the electrodes, provided, of course, that the potential difference between the electrodes is raised in proportion to the shortening of said electrical pulse. Apparently, the only theoretical limit is the voltage drop which might cause arcing or breakdown of the electrical resistance of the layer.

It has also been found that the thickness of the colloid which coagulates is substantially to the amount of current passed through the layer. From this, it follows that a colloid coating of highly uniform thickness can be formed on a flat electrode.

It is one of the objects of the invention to provide a method and a system to coat a metal base with a coating of highly uniform thickness, since the thickness is electrically controlled.

It is another object of the invention to provide a method and system as above in which the coating is produced by electroosmosic drying of a colloid.

It is another object of the invention to provide an image or symbol reproduction method and system using the principle of electroosmosic drying of a colloid which is simple, very fast and very precise.

Another object of the invention is to provide an elec tric reproduction method and system which are suitable for poly as well as monochromatic reproduction.

Another object of the invention is to provide an electric reproduction method and system in which the num ber of separate electrodes required can be reduced to a minimum.

The foregoing and other objects and advantages of the invention will becomes more apparent during the following disclosure and by referring to the drawings, in which:

FIG. I is a cross-sectional view in elevation of an image reproduction system according to a first embodiment of the invention.

FIG. 2 is a perspective view ofan image reproduction system according to a second embodiment of the invention;

FIG. 3 is a cross-sectional elevation view of an image reproduction system according to a third embodiment of the invention;

FIG. 4 is a schematic plan view of the system according to the third embodiment of the invention;

FIG. 5 is a cross-sectional and partial view of one embodiment of the electrode assembly of the system of FIGS. 3 and 4;

FIG. 6 is a cross-sectional and perspective view of a second embodiment of the electrode assembly of the system of FIGS. 3 and 4;

FIG. 7 is a schematic representation of another embodiment of the electrode system; and

FIG. 8 is a view of the single electrode cylinder and image reproduced therefrom.

There will now be described various systems for image or symbol reproduction. The image reproduction method system illustrated in FIG. 1 includes a shallow container or pan 1 arranged to contain a liquid state mixture of a colloid in water, together with an electrolyte. As a colloid, gelatin is preferred and albumin is also suitable.

The electrolyte must have a high conductance when dissolved in water and must cause a minium of gas liberation at the electrode on which the coagulated colloid will adhere.

As the electrolyte potassium chloride performs particularly well, however, the following salts are also suitable, namely; sodium chloride, calcium chloride, nickel chloride, ammonium chloride, copper chloride and manganese sulfate.

An electically conductive plate 3 forming a support constitues a first electrode connected by a conductor 4 to a battery 5 or other equivalent direct current power supply. A frame 6, of suitable thickness, such as preferably between 0.5 to 10 mils, is positioned onto the first electrode constituted by the support plate 3, preferably along the edges thereof.

A matrix 7 is positioned onto the spacing frame 6 coextensive therewith and includes a flat bottom surface 8 which thence is uniformly spaced from the top surface of the support plate 3. A layer 9, of the abovementioned liquid state mixture, is thus trapped between the matrix 7 and the base plate or support 3. The layer 9 therefore has a uniform thickness of 0.5 to 10 mils as determined by the thickness of the spacer frame 6. The layer must be as thin as possible to have sharp dots of coagulated colloid while avoiding arcing between the oppositely biased electrodes. The matrix 7 is formed of a body or plate 10, of electrically insulating material, into which are arranged an array of electrodes constituted of a plurality of thin wires 11, or the like, which adjoin one another without making electrical contact one with any other. Obviously, the plurality of adjoin ing electrodes 11 may be produced in many other ways. such as defined in detail in my co-pending U.S. Pat. ap-

3 plication No. 229.303. filed Feb. 25. 1971 now US. Pat. No. 3,752,746.

A sweeping device l2, of any suitable type arranged to cause sequential connection of the plurality of electrodes 11, is connected in circuit with the battery and a modulator 13 for transmission of an image representing signal to spatially selected ones of the electrodes as determined by the pulsating electric signal. The sweeping device 12 may be of suitable electric or mechanical type. The image representing signal may be produced by any type of image analyzing modulator 13, such as a television camera or a mechanically movable photocell to produce a chain of electrical signals which represent an image in signal form. By appropriately synchronizing the sweeping movements of the modulator 13 and sweeping device 12, the image representing signals may be transmitted to the appropriate electrodes 11 to cause energization of the latter. The signal may be modulated in voltage or time by a modulator to produce a similar energization of the corresponding elec trodes.

When using gelatin or albumin as a colloid, it will coagulate and adhere to the positive electrode, namely to plate 3, as dots opposite those of the electrode 11 which have been biased. The dots have a thickness which varies in accordance with the amount of electricity passed through the biased electrodes and will have a maximum thickness corresponding to the thickness of the layer 9.

It is seen that by suitably varying the amount of electricity passed at each electrical pulse, the half-tones of an image can be reproduced. The layer must be kept at a temperature high enough to maintain the same in liquid state during the passage of electricity.

As an example. a layer 9 has been made of a composition containing 14 grams of geltain (photographic grade); 2 grams of potassium chloride; and 100ml of distilled water. The layer was maintained at 80 F. during the electrical action to keep the gelatin suspension in liquid state.

The layer of the thickness 9 was 2 mils; the diameter of the electrodes II was mils; a voltage of 100 volts was applied between the negative and positive electrodes and the electrical pulses were as short as l/l00,000 of a second and definite gelatin coagulation was observed.

With the above mixture. it has been found that the power to produce coagulation over an area of a square l X lmm was the charge of a capacitor of 50 micro farads at 40 volts. Tests have shown that the electrodes 1], namely the negative electrodes, could be made of any metal but that positive electrode 3 must be made of a metal which will resist electrolytic attack. preferably, stainless steel is used in particular grades 302 and 316; but aluminum and iron electrodes are suitable. Also, the positive electrodes are advantageously made porous to enhance the adherence of the coagulated gelatin thereon.

There was found to be partically no gas generation at the electrodes and. more particularly. at positive electrode 3 and thus gelatin adherence was not in any way impaired by gas generation.

Tests have also been made using the following elec trolytes. namely: sodium chloride. calcium chloride. nickel chloride. ammonium chloride. copper chloride and also manganese sulfate. However, the speed of coagulation obtainable was not as high. probably due to the smaller effective conductance of these salts when in solution in water.

After coagulation of the gelatin. the non-coagulated portion of layer 9 still in liquid state was removed by any suitable means. such as by washing off. airjet or wiping to fully uncover the coagulated gelatin.

After separation of the coagulated portion of the layer 9 from the non-coagulated portion. the coagulate may be set or hardened in any suitable way, such as by irradiation. drying or chemically.

The following briefly outlines some applications of the invention:

1. The coagulated gelatin on its support can be dyed by organic dyes and then the dye in the gelatin may be transferred into the gelatin of a gelatinized sup port. such as paper. to produce copies.

2. The coagulated gelatin after dyeing effected either before or after the electrical bias action, is completely transferred onto an end-use support.

3. The coagulated gelatin is wetted and then covered with a powdered colorant that can be transferred onto an end-use support.

4. The coagulated gelatin is wetted while the interstices or remaining plate area is coated with waterrepellent ink. whereby the plate can be used as lithographic plate.

5. The coagulated gelatin protects its support while the remaining areas are etched to form a photoengraved plate. This method may be used for making printed circuits.

6. The coagulated gelatin areas can produce a cliche for silk screen.

7. The gelatin can be coagulated directly on an enduse positive electrode surface. like paper coated with a white metallic and non-corrodible substance.

8. The composition forming layer 9 can contain a colored powder in solid form. like graphite or carbon black. Upon gelatin coagulation. the powder will become entrapped in the coagulated gelatin on the positive electrode.

9. A non-polymerized resin can be incorporated into the layer 9 and will become entrapped into the coagulated gelatin. Then the latter can be subjected to a treatment for polymerizing the resin in the co agulated gelatin to therefore form a tough coating.

10. Several differently colored images of coagulated gelatin can be transferred onto an end-use support in superposed relation to produce a polychromatic image.

1 l. The layer 9 can be made transparent. that is gelatin not containing any dye or any other colored material and the electrical pulse can be controlled to coagulate gelatin dots of a thickness that will cause light interference on reflective positive electrode. The color of these light interferences can be varied by suitably controlling the thickness of the gelatin dots to one-half the wave length of the desired color. Thus. color prints can be obtained without the use of any dye in accordance with the principle of Lipmann interferential photography.

It is obvious that by replacing the matrix 7 by a single plate-like electrode similar to plate 3 and connecting the same to the positive pole of the battery 5 with the intermediary of a suitable voltage regulator and electrical pulse timer. one can obtain a gelatin coating on plate 3 of highly uniform and controlled thickness.

Thus, one can produce photographic films by incorporating the silver salts into the gelatin layer 9.

Reverting to the image reproduction system, it will be noted that the number of separate negative electrodes can be reduced with respect to the embodiment of FIG. 1 wherein as many electrodes 11 are required as the desired number of dots composing the reproduced image.

The reproduction system schematically represented in FIG. 2 includes a container or trough 14 into which dips, or is immersed, a roller 15 rotatably carried by an axle 16, in any well known manner. A printing head 17 is mounted adjacent the roller 15 and includes a matrix defined by a plurality of wires 18, or the like, forming the negative electrodes and arranged into a row extending lengthwise of the roller and embedded in a mass of electric insulating material. The electrodes 18 are closely spaced, between 0.5 to 2. mills from the cylindrical surface of the roller. At least the surface of the latter is electrically conducting and forms a common electrode which is electrically connected in any suitable manner to the positive pole of a circuit containing battery 5, modulator l3 and sweeping device 12. Roller 15 is rotated at a uniform speed depending on the sweeping speed of device 12 and such that the roller surface is substantially uniformly swept line by line by electrodes 18. With this system, the number of negative electrodes 18 is decreased to a number which is the square root of the required number of negative electrodes in the first embodiment for the same number of resolution points of the image to be reproduced.

In the embodiment illustrated in FIGS. 3 and 4, the support is constituted of a set of conducting plates 19 which are stacked parallel to each other and isolated one from another. such as by dielectric layers. A second set of conducting plates 20 are stacked and insulated as the plates 19 but extend transversely of the latter and in a plane parallel thereto to form the layer 9 between them.

A sweeping device 21 is connected to each set of conducting plates and arranged to sequentially connect the latter in circuit with the battery 5 and the modulator 13. As a result, both sets of conductors 19 and 20 are swept such that, at anyone time, only one conducting plate of each set is in circuit with the battery 5 and the modulator l3 and there results that only the inter section of these two plates is allowed to pass a pulse through the layer 9. If the pulsating electric signal produces a pulsation when the sweeping is at that intersection, a pulse is produced at the corresponding point of the layer 9. As is well known in television. the synchronization of the sweepings of both the image reader and the image writer allows the true reproduction of the image. In this embodiment. the number of electrodes to be contacted by sweeping devices 21 is decreased to twice the square root of the number of resolution points of the image to be reproduced.

' FIG. 5 illustrates the arrangement of the plates 19 and 20 between insulating strips 22 of plastic. rubber or the like to form matrices.

In the embodiment of FIG. 6, the plurality of electrodes are made of wires or rods 23 and 24 which are partly embedded into an insulating body 25.

In the embodiment illustrated in FIG. 7, an image is produced using a different system for sweeping the electrically reactive layer. This sytem includes a drum 26, of any suitable electric insulating material, such as of rubber or plastic around which is secured an outer shell 27 of electrically conductive material suitable to constitute an electrode; in this case, the positive electrode. A shaft 28 is provided to removably mount the drum 26 thereon in abutment between a flange 29 and a securing nut 30.

Another shaft 31 is arranged parallel to the shaft 28 and carries a sweeping drum or roller 32 secured thereon for rotation therewith. Both shafts 28 and 31 and, consequently, both drums are arranged and interconnected in any suitable way for synchronized rotation.

The roller or drum 32 has a helicoidal ridge 33 on the cylindrical surface thereof and forming one turn, a full 360, around the latter, such that for any angular position of the sweeping drum 32, there is one point of the sweeping ridge 33 which is nearest the shell 27 than any other point of the helicoidal ridge. There results that, upon rotation of the sweeping drum 32, the length of the electrode or shell 27 is swept by sucessive points of the ridge 33. The roller 32 and the ridge 33 integrally formed therewith are made of electrically conductive material forming in this case a negative electrode. The ridge 33 clears the outer face of the shell 27 by some 0.5 to 10 mils to allow a very thin layer of the abovementioned substances to pass therebetween. The layer, not shown, may be produced in many different ways which do not form part of the present invention, such as, by immersion in a trough, by a nip roller or edge or by a dispensing roller similar to an inking roller.

Similarly, as for the previously defined embodiments, a battery 5 and modular 13 are connected in series with the opposite electrodes; in this case, the shell 27 and the sweeping roller 32. Roller 34 establish the electrical contacts with the two electrodes.

In the embodiments of FIGS. 1 to 6 inclusive, an image may be produced by subjecting spatially selected ones of the electrodes to pulsations to pass current through the corresponding points of the layer 9 while maintaining the temperature such that the gelatin remains liquid.

In the embodiment of FIGS. 7 and 8, an image may be transferred onto the electrode defined by the shell 27 by appropriate synchronized rotation of the sweeping drum or roller 32 relative to the drum 26, while the negative electrode is subjected to the desired electrical pulsation while the layer of colloid, water and salt is maintained between the ridge 33 and the electrode 27. The pulsations then produce spatially and selectively biased points onto the outer surface of the electrode and through the layer thereon.

These results a coagulation of the gelatin at those points and adherence thereof against the positive electrode. The amount of coagulation at those points may be differentially controlled by correspondingly or proportionally controlling the amplitudes of the pulses, either in voltage or time. which are fed to those points. There results imprints of varying intensities and. therefore. half-tones.

Know computer printing outputs operate at about l/lOOOth of a second per character. whereas such outputs based on the system of the present invention could operate at at least times faster.

What I claim is:

1. An electric reproduction printing method comprising interposing a thin layer of a composition in substan tially liquid state containing water, an electrolyte, and an electrolytically coagulable colloid, between opposite negative and positive electrolytically inert electrode means, electrically and selectively biasing said opposite electrode means with direct current for a relatively short period of time and concurrently sweeping the positive electrode means by the negative electrode means to thereby cause point by point selective coagulation and adherence of colloid onto said positive electrode means, and removing the non-coagulated colloid. whereby the coagulated colloid is representative of a desired image.

2. An electric reproduction method as defined in claim 1, wherein said colloid is selected from albumin and gelatin, and said electrolyte is selected from chlorides and sulfates and said colloid is maintained in the liquid state by controlling the temperature of said layer.

3. An electric reproduction method as defined in claim 2, wherein said layer is adjusted to a thickness of from 0.5 to 10 mils.

4. An electrical reproduction method as defined in claim 3, wherein said negative electrode means includes a plurality of juxtposed electrodes in contact with said layer and said sweeping includes electrically negatively biasing spatially selected ones of said plurality of electrodes with respect to said positive electrode means, passing electric pulses through said layer between said selected electrodes and said positive electrode means to produce said coagulation of the colloid at points opposite to said selectively biased electrodes.

5. An electric reproduction method as defined in claim 3, further including varying the amount of coagulated colloid at the points of coagulation by varying the amplitudes of the electrical biasing concurrently with said sweeping.

6. An electric printing system comprising a thin layer in liquid state and containing colloid selected from gelatin and albumin, water and a salt selected from chlorides and sulfates, negative and positive electrolytically inert electrode means arranged in spaced-apart oppo site relationship in engagement with opposite surfaces of said layer, means to cause sweeping of said positive electrode means by said negative electrode means, and means to electrically bias said opposite electrode means through said layer with direct current for a short period of time during operation of said sweeping means to produce coagulation of the colloid and its adherence to said positive electrode means at selected points of the surface of the latter.

7. An electric printing system as claimed in claim 6, further including means to vary the amplitude of the electrical biasing concurrently with the operation of the sweeping means.

8. An electric printing system as defined in claim 6, wherein said negative electrode means includes a plurality ofjuxtaposed electrodes arranged to contact said layer and said sweeping means is a device connected to said electric biasing means and sequentially connected to said plurality of juxtaposed electrodes to sequentially connect the latter to said electric biasing means.

9. An electric printing system as defined in claim 6, wherein said positive electrode means include a rotatable cylindrical conducting member and said negative electrode means include a rotatable cylinder and a helicoidal conductive ridge on said cylinder and constituting a single electrode, said cylinder and cylindrical conducting member rotatable in opposite directions about parallel axes. whereby said ridge sweeps the surface of said cylindrical conducting member.

10. An electric printing system as defined in claim 6, wherein said electrode means includes a first set and a second set of a plurality of parallel conductors, the conductors of the first set extending transversely of the conductors of the second set and in a plane parallel thereto and said sweeping means sequentially and synchronously connect the conductors of the respective sets to said electric biasing means.

11. An electric printing system as defined in claim 6, wherein said layer is from 0.5 to 10 mils thick.

12. An electric printing system as defined in claim 11, wherein said colloid is selected from albumen and gelatin and said salt is selected from potassium chloride. sodium chloride, calcium chloride. nickel chloride, ammonium chloride, copper chloride and manganese sulfate.

13. An electric printing system as claimed in claim 6, wherein said said positive electrode means has an ac tive surface layer made of a metal selected from stainless steel and aluminum. 

1. AN ELECTRIC REPRODUCTION PRINTING METHOD COMPRISING INTERPOSING A THIN LAYER OF A COMPOSITION IN SUBSTANTIALLY LIQUID STATE CONTAINING WATER, AN ELECTROLYTE, AND AN ELECTROLYTICALLY COAGULABLE COLLOID, BETWEEN OPPOSITE NEGATIVE AND POSITIVE ELECTROLYTICALLY INERT ELECTRODE MEANS, ELECTRICALLY AND SELECTIVELY BIASING SAID OPPOSITE ELECTRODE MEANS WITH DIRECT CURRENT FOR A RELATIVELY SHORT PERIOD OF TIME AND CONCURRENTLY SWEEPING THE POSITIVE ELECTRODE MEANS BY THE NEGATIVE ELECTRODE MEANS TO THEREBY CAUSE POINT BY POINT SEELECTIVE COAGULATION AND ADHERENCE OF COLLOID ONTO SAID POSITIVE ELECTRODE MEANS, AND REMOVING THE NON-COAGULATED COLLOID, WHEREBY THE COAGULATED COLLOID IS REPRESENTTIVE OF A DESIRED IMAGE.
 2. An electric reproduction method as defined in claim 1, wherein said colloid is selected from albumin and gelatin, and said electrolyte is selected from chlorides and sulfates and said colloid is maintained in the liquid state by controlling the temperature of said layer.
 3. An electric reproduction method as defined in claim 2, wherein said layer is adjusted to a thickness of from 0.5 to 10 mils.
 4. An electrical reproduction method as defined in claim 3, wherein said negative electrode means includes a plurality of juxtposed electrodes in contact with said layer and said sweeping includes electrically negatively biasing spatially selected ones of said plurality of electrodes with respect to said positive electrode means, passing electric puLses through said layer between said selected electrodes and said positive electrode means to produce said coagulation of the colloid at points opposite to said selectively biased electrodes.
 5. An electric reproduction method as defined in claim 3, further including varying the amount of coagulated colloid at the points of coagulation by varying the amplitudes of the electrical biasing concurrently with said sweeping.
 6. An electric printing system comprising a thin layer in liquid state and containing colloid selected from gelatin and albumin, water and a salt selected from chlorides and sulfates, negative and positive electrolytically inert electrode means arranged in spaced-apart opposite relationship in engagement with opposite surfaces of said layer, means to cause sweeping of said positive electrode means by said negative electrode means, and means to electrically bias said opposite electrode means through said layer with direct current for a short period of time during operation of said sweeping means to produce coagulation of the colloid and its adherence to said positive electrode means at selected points of the surface of the latter.
 7. An electric printing system as claimed in claim 6, further including means to vary the amplitude of the electrical biasing concurrently with the operation of the sweeping means.
 8. An electric printing system as defined in claim 6, wherein said negative electrode means includes a plurality of juxtaposed electrodes arranged to contact said layer and said sweeping means is a device connected to said electric biasing means and sequentially connected to said plurality of juxtaposed electrodes to sequentially connect the latter to said electric biasing means.
 9. An electric printing system as defined in claim 6, wherein said positive electrode means include a rotatable cylindrical conducting member and said negative electrode means include a rotatable cylinder and a helicoidal conductive ridge on said cylinder and constituting a single electrode, said cylinder and cylindrical conducting member rotatable in opposite directions about parallel axes, whereby said ridge sweeps the surface of said cylindrical conducting member.
 10. An electric printing system as defined in claim 6, wherein said electrode means includes a first set and a second set of a plurality of parallel conductors, the conductors of the first set extending transversely of the conductors of the second set and in a plane parallel thereto and said sweeping means sequentially and synchronously connect the conductors of the respective sets to said electric biasing means.
 11. An electric printing system as defined in claim 6, wherein said layer is from 0.5 to 10 mils thick.
 12. An electric printing system as defined in claim 11, wherein said colloid is selected from albumen and gelatin and said salt is selected from potassium chloride, sodium chloride, calcium chloride, nickel chloride, ammonium chloride, copper chloride and manganese sulfate.
 13. An electric printing system as claimed in claim 6, wherein said said positive electrode means has an active surface layer made of a metal selected from stainless steel and aluminum. 